One way for them to get sick is to have the water temp. alot warmer than the air temp. Such as keeping them in your garage in the winter with a heater in their aquarium. Their body stays warm but when they come up to breathe, the cold air shocks their lungs. I believe its better to brumate them than put them in that kind of enviroment. If you have a good heat lamp to put over the tub this might help the situation but you have to remember that they are nocturnal.